Of the various types of sprinklers used in fire suppression systems, the compressed lever and strut variety finds extensive use. Compressed lever and strut sprinklers use an inherently unstable trigger mechanism comprising a compression member that compresses a strut and lever against a sealing member which closes the outlet of the sprinkler. The strut is positioned lengthwise between the compression member and the sealing member, and the lever is positioned between an end of the strut and the compression member. The strut has a line of action that is offset from the line of action of the compression member. This offset allows the compression member to induce a torque in the lever, which, if unrestrained, would rotate away from the strut, causing the strut to disengage from between the compression member and the sealing member and allow the sprinkler to open. A frangible link extends between the strut and the lever, normally preventing rotation of the lever. The link separates in response to an increase in the ambient temperature indicative of a fire, thereby allowing the lever to rotate and displace the strut from between the compression member and the sealing member, releasing the sealing member and opening the outlet. Fire suppressing liquid may then flow through the sprinkler to contain and extinguish the fire.
For proper functioning of the sprinkler, the trigger mechanism must remain unstable. It is observed however, that a force applied to the free end of lever tending to move the end toward the strut will shift the line of action of the strut, causing it to align with the line of action of the compression member. When this occurs, the mechanism becomes stable, there is no torque on the lever and the link is no longer under tension and is not needed to hold the lever in position. When a temperature increase from a fire causes the link separate, the strut and the lever remain in position between the compression member and the sealing member, holding the sealing member in position closing the sprinkler outlet. In this situation, the sprinkler fails to open and discharge water and the fire propagates. The force applied to the lever which renders the trigger mechanism stable may occur as a result of improper handling during shipping or installation. The mechanism may also be rendered stable due to improper assembly at the factory. It is, furthermore, very difficult to visually detect whether a trigger mechanism is in a stable or an unstable condition. There is clearly a need for an improved sprinkler of the compressed lever and strut type which does not suffer from this disadvantage.